A back monitoring system with a monitor installed in a tractor and a camera installed in a trailer, so that a rearview image behind the trailer captured by the camera is displayed on the monitor in the tractor, is known. In the back monitoring system of this type, if the monitor and the camera are in wired connection with each other, when the trailer is coupled to or decoupled from the tractor, complicated work for physically connecting or disconnecting the wiring cables needs to be done. Therefore, it is preferable that the back monitoring system is configured such that the monitor and the camera are wirelessly connectable to communicate.
Infrastructure mode may be considered as a mode to be employed for connecting two terminal devices (the monitor and the camera) to communicate wirelessly. In infrastructure mode, a terminal device may communicate wirelessly with another terminal device through a predetermined access point. However, when wireless communication connection is established in the back monitoring system in infrastructure mode, it is necessary that a list of IP (Internet Protocol) addresses allocated to each one of all cameras having been shipped is managed and updated by necessity on the monitor side. Moreover, in infrastructure mode, it is necessary that the wireless communication connection is established through an access point; therefore, while the access point intervenes in the communication, displaying the rearview image on a screen of the monitor may be lagged. In these reasons, it is considered to be preferable that a monitor and a camera in a back monitoring system are connected in ad-hoc mode so that they communicate each other directly.
In ad-hoc mode, the terminal devices are enabled to wirelessly communicate each other directly without intervention by an access point. In ad-hoc mode, by eliminating the access point, easy establishment of an ad-hoc network is provided. Meanwhile, however, in ad-hoc mode in general, in compensation for the easy establishment of the ad-hoc network, defects may be found in, for example, a lower security level in device authentication.
As a wireless communication technique to cover this type of defect, a technique to apply WPS (Wi-Fi Protected Setup), which is prescribed for infrastructure mode, to ad-hoc mode is disclosed in, for example, patent document 1 (Japanese translation of PCT international application No. 2010-500817A), patent document 2 (Japanese translation of PCT international application No. 2010-507282A), and patent document 3 (Japanese Patent Provisional Publication No. 2010-251895A). Moreover, recently, Wi-Fi Direct (registered trademark) which introduces a device authorization process in ad-hoc mode has been suggested and practically used.
However, applying ad-hoc mode in these manners to the wireless communication system for the back monitoring system may not always be preferable. For example, in Wi-Fi Direct (registered trademark), similarly to conventional ad-hoc mode, terminal devices are allowed to wirelessly communicate with each other without intervention by an access point. However, in practical use, the terminal devices enabled for Wi-Fi-Direct (registered trademark) are equipped with an access point function so that one of the P2P-connected terminal devices serves as an access point. Therefore, in the case of Wi-Fi Direct (registered trademark), it is necessary to activate the access point function prior to performing the device authentication process, and it may take time to establish the connection. Thus, the inventor provides a wireless communication system, in which connection in ad-hoc mode between terminal devices can be established easily and securely, and a terminal device constituting the wireless communication system.